1. Field of the Invention
The present invention relates to a reception technology and, more particularly, to a receiver and receiving method for receiving an RF signal in a superheterodyne system.
2. Description of the Related Art
A superheterodyne receiver is provided with a frequency synthesizer. The superheterodyne receiver converts a received signal into an intermediate frequency signal by mixing a local oscillator frequency signal successively generated by the synthesizer with the received signal. The superheterodyne receiver searches for the received signal by subjecting the intermediate frequency signal to frequency description. The signal received in a superheterodyne receiver may not be a target signal (desired signal). This is due to the reception of an image signal or a spurious signal. It is therefore necessary to discriminate an image signal and a spurious signal. This is addressed by receiving two channels (local oscillator frequency±intermediate frequency) simultaneously and searching a width equal to twice the frequency of the intermediate frequency, and then shifting the local oscillator frequency by an amount equal to twice the intermediate frequency before searching, again, a width centered at the post-shift frequency and equal to twice the intermediate frequency (see, for example, patent document 1).
[patent document 1] Japanese Patent Application Publication No. Hei7-235886
Suspension of a scan for a reception frequency due to the reception of an image signal requires operating the radio apparatus to resume the scan. The extra job bothers the user of the radio apparatus. Further, the scan is started/suspended repeatedly to search for a reception signal so that the result that the scanning speed is lowered. Further, if a strong signal is found in an adjacent channel, a signal is detected in the adjacent channel in which that signal is found with the result that the scan is suspended.